Galaxy Surprise Sheds Light on Dark Matter

A study of small galaxies circling around the Milky Way found
that while they range dramatically in brightness, they all surprisingly pack about
the same mass. The work suggests there is a minimum size for galaxies, and it
could shed light on mysterious dark matter.

Spinning around the Milky Way are at least 23 pint-sized
galaxies, each shining with the light of anywhere from a thousand to a billion
suns. Though each of these galaxies is very dim compared to large galaxies like
our own, they span a large range in brightness.

Astronomers led by Louis Strigari of the University of
California-Irvine studied the movements of individual stars in these satellite
galaxies to determine the mass of each galaxy.

"What we found was astonishing, which was that they all
had the same mass," said researcher James Bullock, a UC-Irvine
astrophysicist. "It's not what we were expecting ? we were really taken
off guard."

Loaded with dark matter

The finding could help explain the mysterious stuff called
dark matter and how it affects the formation of galaxies. Nobody knows what
dark matter is, but its presence is revealed by gravity that is not produced by
the regular matter that can be seen.

Despite their wide-ranging brightnesses, all of the 23
satellite galaxies around the Milky Way seem to have a central mass of 10
million times that of the sun. And what's more, almost all of that mass seems
to be made up of dark matter, with just the tiniest smidgen of visible matter producing
stars.

"These are by far the dimmest galaxies that have ever
been discovered, and the least luminous of these things are the most
dark-matter-dominated things that we know about," Bullock told SPACE.com.

Though they qualify as galaxies, the satellites are not
pinwheel spirals like the Milky Way and its cousins. Rather, these dwarf
galaxies look more like diffuse, puffy balls of light.

Dark matter wells

The fact that these galaxies, the smallest ever seen, all
weigh about the same may mean that there is a lower threshold for the mass of
galaxies. Though as to why there would be a minimum galactic size, "I have
to say right now we're totally at a loss," Bullock said.

The researchers do have some ideas, though.

For one, maybe there are no dark
matter clumps smaller than these galaxies, and their size represents the
critical mass necessary for dark matter to condense into a clump.

"Maybe we've kind of hit a limit of how dark matter can
cluster, and if that?s true, maybe that tells us something about the dark
matter particle itself," Bullock said.

Another option is that dark matter can form smaller clumps
than these, but it just can't give rise to visible-light galaxies, he said.
Perhaps the process of galaxy formation, which isn't fully understood, depends
on having a minimum mass to begin with.

"You can think of a dark matter clump as a well, and
the more massive the dark matter clump, the deeper that well is, and the harder
it is for the normal matter to float out of it," Bullock said. "It
could be that there are smaller clumps, but their wells are so shallow that any
normal matter just falls right out."

The hunt for dark matter

The astronomers want to do further research, such as a
detailed study of the satellite galaxies around our neighbor, the
Andromeda galaxy, to probe the meaning of their discovery.

"We want to try to figure out if what we're learning is
really a limit on how massive a clump of dark matter can be, or just a limit on
massive a galaxy can be," Bullock said.

The researchers hope that by combining their findings with
new observations and predictions made by theoretical models of dark matter,
scientists may ultimately get to the bottom of what dark matter is made of.

At the very least, the discovery raises many new questions.

"These are very intriguing results, absolutely,"
said Savvas Koushiappas, a physicist at Brown University who was not involved
in the study. "The thing I find extremely puzzling is, well, what does
this tell us about physics? Is it truly a problem with [galaxy formation], or
is it touching something fundamental about dark matter? It's a very interesting
result and deserves attention, and it's something that now we have to think
about. We have a lot of work to do."

The scientists detail their findings in the Aug. 28 issue of
the journal Nature.